As is known in the art, thermal ink jet printing systems include printheads or printbars which utilize thermal energy selectively produced by heating elements located in capillary-filled ink channels near channel terminating nozzles or apertures to vaporize the ink momentarily and from temporary bubbles on demand. The nozzles are typically arranged in a linear fashion either in a column or a row. The rapid formation of a temporary bubble causes an ink droplet to be expelled from the printhead and propelled towards a recording medium. U.S. Pat. No. 4,774,530 to Hawkins describes a configuration of a thermal ink jet printhead. The printhead may be incorporated in either a carriage-type printer or pagewidth type printer, The carriage type printer generally has a relatively small printhead containing the ink channels and nozzles. The printhead is usually sealingly attached to a disposable ink supply cartridge and the combined printhead and cartridge assembly is attached to a carriage and is reciprocated to print one swath of information (equal to the length of the column of nozzles) at a time on a stationary recording medium, such as paper. After the swath is printed, the paper is stepped a distance equal to the height of the printed swath so that the next printed swath is contiguous therewith. The procedure is repeated until the entire page is printed. In contrast, the pagewidth printer includes a stationary printhead having a length equal to or greater than the width of the paper. The paper is continually moved past the pagewidth printhead in a direction normal to the printhead length and at a constant speed during the printing process.
Because the printheads have an arrangement of linearly aligned nozzles, the alignment of the printhead or printbar with respect to the paper is very critical. In printheads which print a single line of pixels in a burst of several banks or segments of pixels each printing a segment of a line, misalignment can be particularly noticeable if not properly aligned. An ink jet printhead having banks of nozzles is described in U.S. Pat. No. 5,300,968 to Hawkins. In these printheads, the banks of nozzles are fired sequentially and the nozzles within a bank are fired simultaneously. Such printheads must be precisely oriented with respect to the process direction so that the printing of the last segment, which is delayed in time from the printing of the first segment, results in a line of pixels that is collinear.
Misalignment of the printhead or printbar with respect to the paper can occur in many ways. For instance, misalignment can occur between the printhead and the cartridge, between the cartridge and the carriage, and even between the carriage and the printer itself. Since each of these instances of misalignment results from differing causes, misalignment problems require different solutions. Some solutions to misalignment are described in the following references.
In U.S. Pat. No. 4,709,245 to Platt optical inspection of the position of an orifice plate on a printhead after mounting on a printer is described. The edges of the orifice plate are detected by moving each printhead past a light source and sensing changes in the reflected light.
U.S. Pat. No. 4,818,129 to Tanuma et al. describes a method for correcting bidirectional printing alignment of a serial dot printer. Correctly aligned printing is obtained through switch operations by making a selection of correction amounts stored in a memory. A switch is pressed by an operator to select the appropriate correction amount and the selected correction amount is written into memory. A test pattern can be printed to show any misalignment and whether further correction is necessary.
U.S. Pat. No. 5,049,898 to Arthur et al. describes a disposable printing assembly which includes a memory element stored with data characterizing the assembly. Alignment data reflecting the alignment of the orifice plate to the printhead is determined and stored in the memory element prior to mounting on a printer. Once mounted, the alignment data is read by the printer. Based on the data, the relative timing of firing signals provide to printhead orifii is adjusted according to the stored information in order to minimize printing errors caused by misalignment.
U.S. Pat. No. 5,289,208 to Haselby discloses apparatus and techniques for aligning the operation of the ink jet printhead cartridges of a multiple printhead ink jet swath printer. An optical sensor includes a quad photodiode detector having outputs which indicate horizontal positions of vertical test lines and vertical positions of horizontal test lines.
U.S. Pat. No. 5,297,017 to Haselby et al. discloses apparatus and techniques for aligning the operation of the ink jet printhead cartridges of a multiple printhead ink jet swath printer. First and second printhead cartridges print non-overlapping horizontal test line segments. An optical sensor detects relative positions of the test line segments. The operation of the first and second printhead cartridges is adjusted to correct alignment.